The present disclosure is directed to methods for reducing the levels of cyclic oligomers produced during the formation of polyetherimide resins. More particularly, a fast and efficient fractionation method is disclosed to reduce the polydispersivity of the polyetherimide resins without having to precipitate the desired polyetherimide in solid form. Another aspect of this invention is to concentrate cyclic oligomers for further use in other applications.
Polymerization reactions typically lead to products of varying polydispersivity or polydispersity, i.e., having a range of components from low to high molecular weight. The quality of a final polymeric product depends to a large extent on how broad its molecular weight distribution is (in most cases, the broader the distribution, the lower the quality). Polydispersivity is expressed as the polydispersivity index (PDI), which is the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn).
In many polymerization reactions, undesirable low molecular weight by-products and unreacted monomers remain in the final product. Such by-products and unreacted monomers can have adverse effects on the properties of the desired polymers and thus must be separated.
For example, aromatic polyethers, particularly polyetherimides, are important engineering resins because of their excellent properties. These polymers may be produced by various methods including the condensation polymerization of a diamine and a dianhydride as in the reaction of m-phenylene diamine (mPD) and bisphenol-A dianhydride (BPADA). The resulting polyetherimides have a polydispersivity of about 2.2.
Alternatively, polyetherimides may be prepared by a displacement polymerization process which reacts salts of dihydroxyaromatic compounds, such as bisphenol A disodium salt (BPA.Na2), with dihaloaromatic molecules. For example, polyetherimides are conveniently prepared by the reaction of salts of dihydroxyaromatic compounds with bis(halophthalimides) as illustrated by 1,3-bis[N-(4-chlorophthalimido)]benzene (hereinafter sometimes “CIPAMI”), which has the structure 
The bis(halophthalimides), in turn, are produced by reacting at least one diamino compound, preferably an aromatic diamine such as mPD or p-phenylenediamine (pPD), and at least one halophthalic anhydride.
According to U.S. Pat. Nos. 5,229,482 and 5,830,974, the preparation of aromatic polyethers may be conducted in solution in relatively non-polar solvents, using a phase transfer catalyst which is substantially stable under the temperature conditions employed. Solvents disclosed in U.S. Pat. No. 5,229,482 include o-dichlorobenzene, dichlorotoluene, 1,2,4-trichlorobenzene and diphenyl sulfone. In U.S. Pat. No. 5,830,974, monoalkoxybenzenes such as anisole, diphenylether, or phenetole are employed. Solvents of the same types may be used for the preparation of the bis(halophthalimide) intermediates.
The general scheme for the production of bis(halophthalimide) and the subsequent production of polyetherimide is set forth in FIG. 1. The polyetherimides produced by these displacement polymerizations have a relatively high polydispersivity, ranging from about 3.6 to about 2.6, depending upon the amount of 3-CIPA and 4-CIPA used in preparing the CIPAMI monomer. Polymers made by these methods can have between about 10% and about 15% of a cyclic monomer by-product.
When bisphenol A, mPD and 4-CIPA are used to produce polyetherimides, it has been found that the level of cyclic oligomers in the final product is about 3%. However, it has been found that the amount of cyclics increases as the level of 3-CIPA is increased as a starting material in CIPAMI synthesis. Where 100% 3-CIPA and mPD are used as the starting material, the amount of cyclic oligomers can range from about 15% to about 20%. Interestingly, it has been found that about two thirds of the cyclic oligomers are a single 1:1 adduct. The reaction scheme demonstrating the use of 3-CIPAMI to produce a polyetherimide with the cyclic oligomer by-product is set forth in FIG. 2.
Other undesirable by-products include short polymer chains and linear oligomers. These by-products, in addition to unreacted monomers, being off specification, must be discarded after separation, increasing the cost and size of the waste stream and reducing the efficiency of the process.
High levels of these low molecular weight species can also have adverse effects on the properties of the resulting polymer. Such negative effects include a lower glass transition temperature (Tg), reduced ductility, and problems with processing including surface appearance, as demonstrated by reduced glossiness.
However, it has also been found that the use of 3-CIPA in combination with other bisphenols and diamines can produce polyetherimides possessing higher Tg (about 15° to about 20° C. higher), and improved flow at high shear. It is therefore desirable to use 3-CIPA as a starting material, at least in part, in the production of polyetherimides.
Means for recovering products from polymerization reactions are known. For example, polymer fractionation processes recover a desired polymer in solid form from a solution by precipitation into an anti-solvent. The process is referred to as total precipitation if the anti-solvent does not dissolve the polymer or low molecular weight species in the polymer such as linear oligomers, cyclic oligomers and monomers. Heptane and other alkanes are examples of anti-solvents which may be used for total precipitation of polymers, especially polyetherimide polymers. However, where such anti-solvents are used, the presence of low molecular weight species in the polymer such as linear oligomers, cyclic oligomers and monomers will result in a product having a higher polydispersivity.
Other methods for recovering polyetherimide polymers include the precipitation of highly polydispersive polymers in toluene, acetone, or tetrahydrofuran, which dissolve low molecular weight species and unreacted monomers from the polymer. Thus, polymers obtained by these methods have reduced polydispersivity.
It is desirable, therefore, to develop a method for preparing polymers which is adapted to the close control of molecular weight and removing unreacted monomers and undesirable by-products by relatively simple means. In the case of polyetherimide polymers, polymers with lower polydispersivity will have improved thermomechanical performance characteristics.